Marine seismic exploration usually involves acquiring seismic data using a seismic acquisition system whose source initiates a down-going seismic wavefield. A portion of the down-going wavefield travels into the underlying earth where it illuminates subsea geologic formations. As it illuminates the interfaces or boundaries between the formations, part of the wavefield is returned (or reflected) back through the earth (propagating in the up-going direction). Part of the reflected wavefield is received by the seismic acquisition system, converted into electrical signals, and recorded for subsequent processing. An analysis of these recorded signals makes it possible to estimate the structure, position, and lithology of subsea geologic formations, thereby completing an important step in the exploration process.
FIG. 1 shows a simplified example of a typical marine seismic acquisition system. A first ship 1 tows a seismic source 2 several feet below the surface 3 of the ocean. The seismic source 2 is activated to produce a down-going wavefield 4d that is at least partially reflected by a subsea formation boundary 5 or subsea impedance discontinuity. The up-going wavefield 4u then travels toward the sensors 6 and is detected.
The sensors 6 used in marine seismic exploration include pressure sensors and velocity (also referred to as xe2x80x9cparticle velocityxe2x80x9d) sensors. Typically, the pressure sensors are hydrophones and the velocity sensors are geophones. The hydrophones measure a scalar pressure and are not sensitive to the propagation direction of the wavefield. The geophones, which may be vertical geophones, provide a vector response measurement whose polarity depends on whether the direction of propagation of the wavefield is up-going or down-going. The amplitude of the geophone response is also related to an angle of the propagation relative to the sensitive direction of the geophone. If a wavefield is recorded by a hydrophone and a geophone with similar electronic impulse responses, then a polarity comparison between the hydrophone and geophone measurement determines whether the wavefield is propagating in the up-going or down-going direction. Hydrophones and geophones disposed at the seafloor are typically used in pairs when collecting seismic data. A combination of this two component or xe2x80x9cdual sensorxe2x80x9d data (pressure and particle velocity) has been useful to cancel down-going multiples from a combined pressure and vertical velocity data signal. The importance of this aspect of the sensor pairing will be discussed in detail below.
In one type of marine seismic surveying, the sensors 6 are located at regular intervals in ocean bottom cables (OBC) 7 that are arranged on the seafloor 9. When necessary, a second ship 8 is used to move the OBC 7 to a new position on the seafloor 9. Several miles of OBC 7 are typically deployed along the seafloor 9, and several OBCs are typically deployed in parallel arrangements. OBC 7 arrangements are particularly well suited for use in certain zones (such as zones cluttered with platforms or where the water is very shallow) where the use of ship-towed hydrophone arrays (not shown) (which are located proximate the ocean surface 3 and are typically referred to as xe2x80x9cstreamersxe2x80x9d) is not practical.
The collection of data with OBC during seismic data acquisition is complicated by secondary wavefields, also known as xe2x80x9cmultiples.xe2x80x9d Multiples comprise trapped water bottom multiples, source side peg-leg multiples, and receiver side peg-leg multiples. Multiples can mask the seismic data of interest, and they amplify and attenuate certain frequencies, thereby complicating the analysis of the recorded signals. The xe2x80x9cmultiple problemxe2x80x9d is caused by, among other factors, the air-water interface at the surface of the ocean or water column. The following discussion provides a more detailed description while implicitly assuming one dimensional geometry.
When the seismic source is fired, the direct arriving down-going wavefield impacts the seafloor. A portion of the down-going wavefield travels into the subsurface and provides the primary seismic data by reflecting off of subsurface formations. Another portion of the same down-going wavefield is reflected back into the water column. This up-going wavefield travels back to the ocean surface and is reflected back in a down-going direction. A down-going wavefield reflected off of the ocean surface may be referred to as a xe2x80x9cghost.xe2x80x9d A ghost subsequently impacts the seafloor where, as for the direct arriving down-going wavefield, a portion travels into the subsurface and a portion is reflected back into the water column to generate subsequent ghosts. Hence, some portion of a ghost is reflected back into the water column and remains trapped in the water column, forming the trapped water bottom multiple (or subsequent multiple ghost arrivals), while the remainder propagates into the subsurface, leading to the formation of delayed and scaled copies of the primary seismic data (referred to as source side peg-leg multiples) as the delayed down-going wavefield reflects off of the subsurface formations.
Up-going wavefields from the subsurface result from the portion of the direct arrival that initially travels into the subsurface (the primary reflection) and the subsequent source side peg-leg multiples that pass through the seafloor. The up-going wavefields will be recorded at the seafloor. However, after being recorded, the up-going wavefields continue upward and subsequently impact the air-water interface and are reflected in a down-going direction. As a result, the primary and source side peg-leg multiples form down-going ghosts. The water trapped portions of these wavefields are called the receiver side peg-leg multiples, and the portion of these wavefields that travel into the subsurface are ignored for the purposes of this discussion because they contain higher order subsurface reflections than are relevant for the analysis presented herein.
FIG. 2 shows two-dimensional examples of wavefields that are produced by a source 10 and are detected by a sensor pair 11. The source 10 is typically located proximate the ocean surface 12. A direct arrival 18 is a wavefield that travels directly from the source 10 to the sensor pair 11. A receiver-side peg-leg 13, which may also be referred to as a xe2x80x9creceiver side multiple,xe2x80x9d is produced when the wavefield is first reflected by a subsurface formation 16 and then by the ocean surface 12 before being detected by the sensor pair 11. A source side peg-leg 15, which may also be referred to as a xe2x80x9csource side multiple,xe2x80x9d is produced when the wavefield reflects off of the seafloor 14, off of the ocean surface 12, and then off of a subsurface formation 16 before being detected by the sensor pair 11. These wavefields differ from a primary wavefield 17 that reflects off of the target formation 16 and is then detected by the sensor pair 11 before experiencing any additional reflections. The water trapped multiple 19 is first reflected off the seafloor and then off the ocean surface before being detected by the sensor pair 11. For all of these multiples, there may be many reverberations in the water column, but no more than one two-way travel path in the subsurface (for the water trapped multiple 19, there is no travel path in the subsurface). Detection and proper processing of the primary wavefield 17 is an important objective in seismic exploration. The primary wavefield 17 may be corrupted by the multiples that may also be detected by the sensor pair 11.
The elimination of multiples can be an important part of obtaining good OBC data because, unlike a towed streamer where surface multiples produce notches in the frequency spectrum that lie beyond the usable bandwidth of the seismic energy, multiples in the OBC data produce notches within the usable bandwidth. The effectiveness of the removal of the multiples is dependent upon how well the pressure and velocity data are matched, so that up-going radiation is identically recorded by the hydrophone and geophone, and on how well the water bottom reflection coefficient (a coefficient representing how well wavefields are reflected by the seafloor) is estimated.
Prior attempts have been made to remove notches in the frequency spectrum. Barr and Sanders, in xe2x80x9cAttenuation of water-column reverberations using pressure and velocity detectors in a water bottom cable,xe2x80x9d Expanded Abstracts of the 59th Annual SEG Meeting (1989, vol. 1), disclose a theory that assumes that the pressure and velocity transduction coefficients are known and are used to match the amplitude and phase response of the hydrophone and geophone. Calibration shooting is used to provide geophone scalars for combining the hydrophone and geophone data. The scalars are equal to:       1    +    R        1    -    R  
where R is the ocean bottom reflection coefficient at each respective receiver station.
U.S. Pat. No. 4,979,150 issued to Barr discloses a system for reducing reverberation noise by applying a scale factor to the pressure and/or particle velocity. An enhanced signal is generated by multiplying at least one of either the pressure or the velocity by the scale factor and then summing the scaled velocity and scaled pressure.
U.S. Pat. No. 5,365,492 issued to Dragoset, Jr., discloses a method wherein geophone noise is adaptively estimated from velocity and pressure. The polarity of the noise is then reversed, and the noise is added to the velocity to form a clean, refined velocity signal. A scale factor is then applied to the refined velocity signal and the pressure is summed with the scaled refined velocity signal. The summed signal is auto-correlated, and a xe2x80x9cvarimaxxe2x80x9d function is computed. The procedure is repeated by incrementing the scale factor until the varimax function most closely approaches unity. This algorithm was developed to determine optimum geophone scalars and reflection data.
U.S. Pat. No. 5,621,700 issued to Moldoveanu discloses a method that involves adding the product of the pressure times the absolute value of velocity with the product of the velocity times the absolute value of the pressure. The method relies on a polarity flip in the velocity between up-going and down-going wavefields.
U.S. Pat. No. 5,835,451 issued to Soubaras discloses a method that combines hydrophone and calibrated geophone signals to eliminate a water trapped multiple. A calibration function is determined by selecting a time window beyond the duration of the direct arrival (e.g., where a source pulse is zero) and minimizing a xe2x80x9ccross-ghostedxe2x80x9d difference between the hydrophone and calibrated geophone recording. Multiples are then attenuated from the combined signal using predictive deconvolution methods that involve making an estimate for a water bottom reflection coefficient.
One aspect of the invention is a method for removing trapped water bottom multiples, receiver side peg-leg multiples, and source side peg-leg multiples from dual sensor OBC data that includes a pressure signal and a velocity signal. The pressure and velocity signals are compared to determine if polarity reversals exist between them. Polarity reversals are used to identify and separate up-going wavefields from down-going wavefields in the pressure and velocity signals. A matching filter is applied to a portion of the velocity signal where polarity reversals exist. The down-going wavefield is estimated by determining a difference between a portion of the pressure signal where polarity reversals exist and the portion of the velocity signal where polarity reversals exist and applying a scaling factor to the result. An attenuated up-going pressure wavefield is then determined by combining the estimated down-going wavefield and the pressure signal.
In another aspect, the invention is a method for removing trapped water bottom multiples, receiver side peg-leg multiples, and source side peg-leg multiples from dual sensor OBC data that includes a pressure signal and a velocity signal. The pressure and velocity signals are compared to determine if polarity reversals exist between them. Polarity reversals are used to identify and separate up-going wavefields from down-going wavefields in the pressure and velocity signals. A matching filter is applied to a portion of the pressure signal where polarity reversals exist. The down-going wavefield is then estimated by determining a difference between the portion of the pressure signal where polarity reversals exist and a portion of the velocity signal where polarity reversals exist and applying a scaling factor to the result. An attenuated up-going velocity wavefield is then determined by combining the estimated down-going wavefield and the velocity signal.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.